scripts/create-pull-request: fix git request-pull syntax

* at least with git 2.6.3 I see git request-pull failing when there is only :{BRANCH} as ending commit

* $ git request-pull origin/dizzy git://git.openembedded.org/openembedded-core-contrib jansa/dizzy-backports:jansa/dizzy-backports The following changes since commit 7bb182bdd130266100fc541fd09b82d09c51cd80:

    build-appliance-image: Update to dizzy head revision (2015-09-29 14:56:04 +0100)

...
And finds correct 7 changes there

* $ git request-pull origin/dizzy git://git.openembedded.org/openembedded-core-contrib :jansa/dizzy-backports
warn: No match for commit 6068d1c90336ddc1fb32856efd1d9ccf07733896 found at git://git.openembedded.org/openembedded-core-contrib
warn: Are you sure you pushed 'jansa/dizzy-backports' there?
The following changes since commit 97756472d3a69eaca95d105494ffea78c6b077e0:

    build-appliance-image: Update to dizzy head revision (2014-10-18 16:16:27 +0200)

...
and lists all commits in _current_ branch since origin/dizzy, then it refuses to continue, because there are too many changes.

* 6068d1c90336ddc1fb32856efd1d9ccf07733896 is this commit in jansa/master-submitted
branch so it really shouldn't be included in pull request from jansa/dizzy branch.
* git help says:
<end>
Commit to end at (defaults to HEAD). This names the commit at the tip of the history you are asking to be pulled.

When the repository named by <url> has the commit at a tip of a ref that is different from the ref you have locally, you can use the <local>:<remote> syntax, to have its local name, a colon :, and its remote name.
* maybe the syntax got changes since git 2.1.0 when Saul added :${BRANCH}
* I haven't found how to respect ${COMMIT_ID in the new syntax

---
 scripts/create-pull-request | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index a3744cc..dd66dfe 100755
@@ -203,7 +203,7 @@ NEWER_GIT_VERSION=210
 if [ $GIT_VERSION -lt $NEWER_GIT_VERSION ]; then
 	git request-pull $RELATIVE_TO $REMOTE_URL $COMMIT_ID >> "$PM"
 else
-	git request-pull $RELATIVE_TO $REMOTE_URL :$BRANCH >> "$PM"
+	git request-pull $RELATIVE_TO $REMOTE_URL $BRANCH:$BRANCH >> "$PM"
 fi
 if [ $? -ne 0 ]; then
 	echo "ERROR: git request-pull reported an error"

2.6.3